Color filter arrays are employed in combination with sensors to define color images or in combination with display devices to permit color images to be viewed. A common approach to producing color filter arrays has been to use organic dyes embedded in a layer which has been patterned by various techniques to render the appropriate filter pattern. This approach has two significant disadvantages. The spectral characteristics of the filter are controlled by the absorbance curves of the dye and layer materials. Altering the spectral characteristics, therefore, requires altering the dye or layer material, which can be a difficult and time consuming process. Furthermore, the dyes may be subject to fading with time especially under harsh environmental operating conditions such as high light levels, etc.
An alternative, which overcomes the disadvantages of the organic dye approach, has been to produce color filter arrays from interference filters made up of alternating layers of two dielectric materials with different refractive indices. Various combinations of pairs of dielectric materials and deposition and patterning techniques have been used. One particular interference filter which uses a metal and dielectric layer is known as a Fabry-Perot interference filter.
In some sense, there exists a synergistic relationship between the choice of dielectric materials and the lithographic patterning technique employed. Thorium fluoride, zinc sulfide and cryolite are often chosen because these materials exhibit good film properties and adhesion even when deposited at temperatures that are compatible with organic based resist systems. Kramer and Hoffman reported in the Journal of Imaging Technology, Vol. 12, No. 5, pages 270-279 (October 1986) on the production of a linear color filter set on a glass substrate. Cryolite and zinc sulfide where chosen as the low index and high index materials, respectively. These materials were selected so that the substrate temperatures could be kept low to prevent damage to the photoresist material. Sulfur from the zinc sulfide, however, has the potential of poisoning an electronic device. This is especially true when filters are fabricated directly onto a device. The filters were based upon an all dielectric Fabry-Perot broad bandpass design. Thirty-seven to forty-one alternating layers of material were deposited. To prevent stress cracking in the final filter, the use of several air anneal steps were required. The process required 2-3 days of preparation time for each filter and resulted in final filter thicknesses of 4.2 .mu.m. Problems with chipping of the filter edge indicates that the resist cross-section may have been trapezoidal rather than the more desirable reentry profile typically used in lift-off processes and suggest that a "pseudo-lift-off" process was employed. See the next section for a discussion of this process. Using metal and a dielectric material (i.e., silver and silicon dioxide), Fabry-Perot interference filters are described in the above referenced McGuckin and Cunningham patent application.
The deposition of titanium dioxide and silicon dioxide stacks onto glass by sputtering techniques has been reported by Shimomoto et al (see 86 Surface Science 16 (1979). For their applications, a subsequent electrode deposition step was conducted at 500.degree. C. The filters were sputtered to control the films and ensure that the filters would survive the electrode deposition step without any change in the spectral characteristics. A lift-off technique was required which could tolerate the temperatures encountered. This was accomplished using a double metal lift-off process employing aluminum and chrome. This process also involved patterning with an organic based resist system followed by transfer of the pattern into the metals. Cyan, yellow and magenta strip filters were produced with thicknesses of 1.2, 0.67 and 1.85 .mu.m, respectively. Green and cyan dichroic filters have been made on glass using titanium dioxide and silicon dioxide dielectric stacks using photoresist as an etch mask. Using positive-working photoresist and dry etching techniques, areas of the yellow and cyan stack are removed to give a cyan and white filter.
The refractory oxides (i.e., silicon dioxide and titanium dioxide) represent good choices based on chemical compatibility with electronic devices and processing. However, these materials are usually deposited at significantly greater temperatures than desirable for a final manufacturing step. Excessive heating may damage the metallizations used for electrical conduction in the device. Therefore, the general approach has been to fabricate filters from these material onto glass substrates and attach the filters to devices using adhesives. Pattern transfer techniques may also be required making the process complicated and cumbersome. Lower temperature deposition techniques for these materials are not known.
Another technique employs a "pseudo-lift-off" process for patterning of brittle, dielectric materials. Conventional positive-working photoresists are lithographically patterned in the usual manner. The filter materials are deposited on top of the resist and onto the substrate through the openings in the resist. Immersion into a solvent removes the resist and unwanted filter areas by a cohesive failure mechanism. The technique relies upon the materials cracking along the edges of the resist pattern and is an inherently unreliable process.
Lithographic techniques based upon removing unwanted areas using photoresist as an etch mask are known. For etch processes, however, there are a number of problems which must be overcome. Chemistries which will attack both dielectric materials at comparable rates are needed. Then a masking material must be found which is compatible with that etch. Additional lithographic steps may be required to pattern the masking material. The process of patterning by etching also does not lend itself readily to changes in dielectric materials or deposition techniques.
Compared with etch processes, a lift-off process represents a good general purpose technique and offers some advantages in process simplicity. To achieve the maximum process control and resolution, it is desirable that a overhanging or reentry resist sidewall profile be generated. This more traditional technique has been used to pattern filters. However, this requires a resist lift-off process which can be coated thicker than the thickest filter. Typical thicknesses for dielectric stack filters are greater than 1 .mu.m and usually range between 2 and 4 .mu.m. Unfortunately, most lift-off processes which produce reentry sidewall profiles have been developed for metallization purposes where the resist coatings are 1 to 1.5 .mu.m thick.
These types of lift-off processes can be categorized into four groups. Some lift-off systems are based upon combinations of light sources and chemistries which photochemically result in retrograde resist edge profiles after development. Image reversal techniques produce similar profiles. In image reversal, exposed areas are chemically altered to decrease solubility. Unexposed areas are subsequently exposed and developed away. The most widely employed technique is known as the chlorobenzene process. By treating a resist coating with chlorobenzene it is possible to alter the dissolution characteristics of the surface such that the overhang structure is produced during development. Silylation techniques have also been used which modify the etching characteristics of a resist surface. Multi-layer resist technology is an area that has received much investigation. Consequently, many permutations using two or three layers of different materials have been reported. A review article by Frary and Seese, Semiconductor International, pages 72-88 (December 1981), discusses the various approaches that have been explored. Of particular relevance is the discussion of structures comprising two layers of positive-working photoresist. The application of one resist onto another suffers from intermixing of the two similar materials such that the development characteristics gradually change throughout the layers; consequently it is difficult to produce the desired overhang structure. Plasma etch or thermal treatment has been used to alter the surface characteristics of the bottom resist layer to produce a "buffer" layer which prevents intermixing. This process allows the top resist layer to be coated uniformly and maintains the distinction between the two layers. Two resist materials may be chosen such that they either exhibit different dissolution rates in the same developer or else they use mutually exclusive developers. In this case, an overhang structure can be produced. Depending upon the treatment conditions used to form the buffer, it may be necessary to use a two step development process with an intermediate etch step to remove the buffer layer.
In terms of ease of manufacturability, the technique used for patterning dichroic filters should be as simple and robust as possible. In other words, the number of processing steps and critical process controls should be minimized. In addition, there should be a wide margin in control factor settings which still result in acceptable product. Etching processes, as indicated previously, require specific etch chemistry and material choices. Suitable masking and etch stop materials must be found that are compatible with the aforementioned etch chemistry. This usually means a pattern transfer process just to produce the appropriate pattern in the etch mask material. Dry etching techniques also have many process controls that have to be monitored and maintained in a manufacturing environment.